1. Field of the Invention
The present invention relates to a ceramic circuit board in which a metallized wiring layer is formed on a ceramic substrate, and more particularly to a ceramic circuit board in which a flow of a large electric current is allowed.
2. Description of the Related Art
As a circuit board on which electronic components such as a semiconductor element are mounted and connected, a ceramic circuit board has conventionally been in wide use that is constituted by forming a plurality of metallized wiring layers on a ceramic substrate.
In such a ceramic circuit board, commonly, the ceramic substrate is made of sintered aluminum oxide or mullite, and a plurality of metallized wiring layers are each made of a metal material having a high melting point, such as tungsten, molybdenum, or manganese. The following is an example of methods for manufacturing the ceramic circuit board. In a case where the ceramic substrate is made of sintered aluminum oxide, firstly, a suitable organic binder, plasticizer, or solvent is mixedly added to powder of a starting material such as aluminum oxide, silicon oxide, magnesium oxide, or calcium oxide, to form a slurry. The slurry is then formed into a plurality of ceramic green sheets in accordance with conventionally-known tape forming techniques, such as a doctor blade method or calendar roll method. Secondly, a suitable organic binder, plasticizer, or solvent is mixedly added to high melting point metal powder, such as tungsten or molybdenum, to form a metal paste. The metal paste is then printed onto the ceramic green sheet in a predetermined pattern in accordance with a printing technique such as a screen printing method. Lastly, the ceramic green sheets, onto which the metal paste is printed in a predetermined pattern, are vertically stacked one upon another as required and are then fired in a reducing atmosphere at a temperature of approximately 1600xc2x0 C. As a result, the ceramic green sheets and the metal paste are formed in one piece by sintering, thereby realizing the ceramic circuit board.
Note that the metallized wiring layer typically has its exposed surface coated with a metal material of certain thickness that is highly corrosion-resistant and exhibits excellent wettability with respect to an adhesive material such as solder (nickel, for example) in accordance with a technique such as a plating method. By doing so, it is possible to effectively prevent corrosion due to oxidation and to allow an electronic component such as a semiconductor element to be firmly connected to the metallized wiring layer with use of an adhesive agent such as solder.
However, the conventional ceramic circuit board has the following disadvantage. The metallized wiring layer is made of a high melting point metal material such as molybdenum or tungsten. Since molybdenum and tungsten have a resistivity which is as high as 5.2xc3x9710xe2x88x926 xcexa9xc2x7cm or above (molybdenum: 5.2xc3x9710xe2x88x926 xcexa9xc2x7cm; and tungsten: 5.64xc3x9710xe2x88x926 xcexa9xc2x7cm), the metallized wiring layer has high electrical resistance. Therefore, in the ceramic circuit board on which an IGBT (Insulated Gate Bipolar Transistor) or power-FET (Field Effect Transistor) is mounted, when a large electric current of approximately 10 A is applied to the metallized wiring layer, intense Joule heat is generated, and the metallized wiring layer suffers from fusion. As a result, the ceramic circuit board is no longer capable of functioning properly.
In an attempt to solve the drawback described above, the metallized wiring layer has been made larger in thickness and width. However, this causes an increase in stress developed in the inner part of the metallized wiring layer, resulting in a crack or break. Furthermore, it is inevitable that the ceramic circuit board as a whole has an unduly large size.
The invention has been accomplished in view of the above-described drawbacks with the conventional art, and accordingly it is an object of the invention to provide a compact ceramic circuit board which admits of a flow of a large electric current.
The invention provides a ceramic circuit board comprising:
a ceramic substrate;
a plurality of metallized wiring layers formed on the ceramic substrate; and
a metal circuit plate made of copper, which is attached to part of the metallized wiring layers,
wherein a condition: Sxe2x89xa76xc3x9710xe2x88x925 i2 is fulfilled, wherein S is a sectional area (mm2) of the metal circuit plate and i is a value (A) of a flowing electric current.
According to the invention, a copper-made metal circuit plate having a low resistivity is placed in the region of the metallized wiring layer through which a large current of approximately 10 A flows, and the metal circuit plate is so designed as to satisfy a condition: Sxe2x89xa76xc3x9710xe2x88x925 i2, wherein S represents the sectional area (mm2) and i represents the value of the flowing electric current (A). Therefore, even if a large electric current of approximately 10 A is applied to the metallized wiring layer, the electric current is allowed to flow smoothly therethrough via the metal circuit plate, and simultaneously generation of intense Joule heat can be prevented effectively. As a result, the ceramic circuit board is capable of functioning properly and stably for a longer period of time.
The invention further provides a ceramic circuit board comprising:
a ceramic substrate;
a plurality of metallized wiring layers formed on the ceramic substrate; and
a metal circuit plate made of aluminum, which is attached to part of the metallized wiring layers,
wherein a condition: Sxe2x89xa79xc3x9710xe2x88x925 i2 is fulfilled, wherein S is a sectional area (mm2) of the metal circuit plate and i is a value (A) of a flowing electric current.
According to the invention, an aluminum-made metal circuit plate having a low resistivity is placed in the region of the metallized wiring layer through which a large current of approximately 10 A flows, and the metal circuit plate is so designed as to satisfy a condition: Sxe2x89xa79xc3x9710xe2x88x925 i2, wherein S represents the sectional area (mm2) and i represents the value of the flowing electric current (A). Therefore, even if a large electric current of approximately 10 A is applied to the metallized wiring layer, the electric current is allowed to flow smoothly therethrough via the metal circuit plate, and simultaneously generation of intense Joule heat can be prevented effectively. As a result, the ceramic circuit board is capable of functioning properly and stably for a longer period of time.
In the invention, it is preferable that the metal circuit plate has a surface plated with a layer that is made of a nickel-phosphorus amorphous alloy containing phosphorus in an amount of 8 to 15 wt %.
In the invention, it is preferable that the ceramic substrate has a thickness of 700 xcexcm or less, and that the plate layer has a thickness ranging from 1.5 to 3 xcexcm.
In the invention, it is preferable that the metallized wiring layer is made of a silver-copper eutectic alloy added with at least one of titanium, zirconium, hafnium, and hydrides thereof.